Internal combustion engines require continuous lubrication for proper performance. As the lubricating oil in an engine oxidizes, however, it becomes acidic. The acidic oxidation products can attack various metals used in the bearings, bushings, connecting rods, pins, and other engine parts, and lead to failure of the engine. In order to prevent this from occurring, basic additives are mixed into most modern lubricating oils. The amount of basic additive that is present in the oil, and that is available for neutralizing the acids that form, is given by the total base number. A new lubricating oil might have a total base number of about 3 to about 15, but, as the basic additives become depleted, the total base number is lowered. Once the basic additives are completely depleted or reduced to a dangerously low level, the quantity of acid and corrosion oxidation products increases rapidly and the attack on the metal parts of the engine proceeds at a rapid pace. Most operators of large fleets of trucks test the total base number of the engine oil at periodic intervals. When the total base number has fallen to about 1 or 2, the oil is changed.
The total base number of the engine oil is determined by removing a sample of the engine oil and having the sample analyzed at an oil analysis laboratory. The disadvantage of this procedure is that there is usually a delay of one or two days between the time the sample is taken and the time that the analysis is returned to the maintenance shop. In the meantime, the truck is typically put back into service and may not be returned to the maintenance shop for an extended period of time. During that time, the basic additives could become further depleted, resulting in severe internal damage to the engine. The alternative of simply changing the engine oil at frequent intervals without performing the analysis is considered to be too expensive for diesel engines, which may contain 3 to 7 gallons of oil.